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The rules governing transfer of power in the secretive communist nation are unclear, but it is widely believed that Kim's youngest son, Kim Jong Un, will succeed him. Facts about North Korea »

In April, the North Korean leader named Kim Jong Un and brother-in-law Chang Sung Taek to the country's powerful National Defense Commission. Chang, who is married to Kim Jong Il's sister, has effectively run the country in the wake of the leader's health problems, according to TIME Magazine.

Chang is "the bridge from Kim Jong Il to Kim Jong Un," according to Baek Seung Joo, who watches North Korea at the Korea Institute for Defense Analysis, who spoke to TIME Magazine last month.